Many industrial applications require heating materials to high temperatures for melting, heat treating, and the like. Heat is often provided by combusting hydrocarbon fuels. However, in these applications the need can arise for supplying heat at different heating rates at different times. Conventional approaches to this need can involve heating the material to a desired high temperature, then discontinuing the combustion in order to let the temperature of the material decrease, and then recommencing combustion when the temperature drops enough that additional heat must be applied. Such “on/off” operation is inefficient in its consumption of fuel and oxidant, and it risks generating unacceptable levels of undesirable byproducts such as nitrogen oxides. Also, it risks imposing thermal stresses on the material by the cycling of the temperature and/or operational stresses on the valves and burners that are repeatedly forced to open and close as the combustion is stopped and started. Other approaches, such as providing two separate burner systems each adapted for a particular type of combustion, with only one system operated at a time, are expensive and take up space.
Therefore, there remains a need for methods and apparatus that enable more efficient and more environmentally tolerable heating of materials, especially under conditions in which the amount of heating is to vary over time.